Confocal, multiphoton, and sectioning microscopes are capable of producing 3-dimensional (3D) images of a volumetric specimen. These images require that the device acquire a series of optical sections from a range of focusing depths. That is, a series of optical sections in the Z-plane. In most systems, focusing in the Z-plane is performed by mechanically moving the specimen relative to the objective lens, or vice-versa. Moving either the microscope stage or the objective lens has two major drawbacks. First, scan speeds in the Z-plane accomplished by mechanical manipulations are slow; second, the scanning movements of the stage in the Z-plane can (and do) disturb the specimen during the imaging process. Focusing deeper into the sample by manipulating the objective lens yields a slow Z-axis scan speed because the objective lens is relatively massive. Once set in motion, it takes a relatively long time period for the objective to settle prior to the next data points being acquired. Manipulating the sample itself is also problematic because the samples are often living cells, and the area of interest to be imaged is a very small portion of the cell. Moving the specimen can disturb the intra-cellular structures that are in the process of being imaged. This results in sub-par micrographic images. Thus, there remains an unmet need for an alternative focusing method that does not require mechanical movement of either the specimen or the objective lens. See, for example, Botcherby et al. (Jul. 15, 2007) “Aberration-free optical refocusing in high numerical aperture microscopy” 32(14):2007-2009, incorporated herein by reference.